Blue light-emitting diodes have been extensively used in practice as solid-state light-emitting elements that use a nitride semiconductor. Meanwhile, ultraviolet light-emitting diodes (UV LEDs) using a nitride semiconductor have also been developed as solid-state light-emitting elements that emit a light ray falling within the ultraviolet range with a shorter wavelength (i.e., an ultraviolet ray). In the ultraviolet range, an ultraviolet ray falling within a deep ultraviolet range of 350 nm or less, inter alia, an ultraviolet ray falling within a wavelength range of about 260 nm to about 280 nm that forms part of a UV-C wavelength range, among other things, is expected to be used in a broad variety of applications, including sterilization, water disinfection, and medical treatment. Thus, to meet such a demand, LEDs emitting an ultraviolet ray falling within the UV-C wavelength range (i.e., so-called “deep ultraviolet LEDs”) are also under development. A deep ultraviolet LED with a typical configuration uses either a sapphire substrate or an AlN single crystal substrate, and includes a multilayer structure of aluminum gallium nitride (AlGaN) based semiconductors including aluminum (Al), gallium (Ga), and nitrogen (N) as major constituent elements and also sometimes including indium (In) as an additional element. Research and development are under way to further increase the output of deep ultraviolet LEDs. As a result of such efforts, deep ultraviolet LEDs, operating at an ultraviolet ray output of about 10 mW, have also been used in practice these days.
One of technical challenges confronted by deep ultraviolet LEDs is that their luminous efficacy still has room for improvement. Indices to the luminous efficacy include external quantum efficiency (EQE). The external quantum efficiency is defined by dividing the number of photons released per unit time from a deep ultraviolet LED into the external space by the number of carriers per unit time that form a drive current. This external quantum efficiency is represented by the product of the three factors, namely, internal quantum efficiency (IQE), carrier injection efficiency (INJ), and light extraction efficiency (LEE). That is to say, if the external quantum efficiency, internal quantum efficiency, carrier injection efficiency, and light extraction efficiency are represented by ηEQE, ηIQE, ηINJ, and ηLEE, respectively, the following equationηEQE=ηIQE×ηINJ×ηLEE is satisfied.
As a result of the development that has been carried out extensively, among those three factors of deep ultraviolet LEDs, the internal quantum efficiency has been improved significantly, and the light extraction efficiency has also been enhanced. Thus, improvement of the carrier injection efficiency is an urgent task to solve. In the case of deep ultraviolet LEDs, in particular, p-type AlGaN with an increased aluminum mole fraction is adopted as a material for a p-type layer in order to widen the bandgap determining the photon energy of emission and to improve the light extraction efficiency by increasing the transmittance of non-light-emitting layers to an ultraviolet ray emitted from the light-emitting layer. Such a deep ultraviolet LED, however, has carrier injection efficiency which is difficult to improve. This problem arises partly because an Mg acceptor exhibits too high activation energy and too intense self-compensation effect in a p-type layer to increase the hole concentration easily.
One of the techniques for improving the carrier injection efficiency by overcoming the problem involved with the use of p-type AlGaN to form a pn junction is to interpose an electron block layer (EBL), of which the composition provides a wide bandgap, between the light-emitting layer and the p-type contact layer. A technique for adopting a multi-quantum barrier (MQB) layer as a form of the EBL has also been known. In a deep ultraviolet LED, among other things, interposing the MQB layer produces multiple reflections of electron matter waves, which contribute to increasing a substantial height of a barrier. Thus, even if such a p-type AlGaN layer, of which the composition has an aluminum mole fraction that is too high to increase the hole concentration and eventually the barrier height of the EBL easily, is adopted, it is still easy for such a deep ultraviolet LED to ensure the EBL function (see Patent Literature 1).